Are fuses always faster than breakers?

Status
Not open for further replies.
iwire

iwire

Moderator
Staff member
Location
Massachusetts
On another forum the topic of fuses came up and quite a few folks feel fuses can operate faster than breakers.

Can a breaker open as fast as fuse?

Do fuses typically open in a 1/2 cycle?

Can breakers open in a 1/2 cycle?

My thought was it depends on the breaker but I may well be wrong and will have to admit it.

I really hate to do that. :p
 
Fuses are not always faster than breakers. Bussmann would like for us to think that fuses take care of all incident energy issues, but if the fuse is not in its current limiting range of fault current, it will sometimes be slower than the instantaneous of the CB.
 
iwire said:
On another forum the topic of fuses came up and quite a few folks feel fuses can operate faster than breakers.

Can a breaker open as fast as fuse?

Do fuses typically open in a 1/2 cycle?

Can breakers open in a 1/2 cycle?

My thought was it depends on the breaker but I may well be wrong and will have to admit it.

I really hate to do that. :p
Click to expand...
Yes, it depends on the breaker. Also the fuse type. The fault current.
I just checked the fuse curve for a standard 10A fuse. It operates in 0.1 sec at 60A.
The standard breaker we use (C curve here) operates in 0.1 sec at 100A.

FWIW, I have never managed to find a breaker with a lower i2t let through than an equivalently rated semiconductor fuse.
I have the full set of Bussmann fuse curves on this computer, so I might check that out later.

So, I think the short answer is what you said. It depends.
 
iwire said:
Are there circuit breakers available that provide the same level of protection as a type J fuse?
Click to expand...
There may be.

What level of protection are you asking about? (e.g. clearing time, I?t, or Ipeak)

UL has had 'listings' for current limiting circuit breakers for decades.
Over the years, there have several instances of circuit breakers that limited I?t or Ipeak, but not both as required by the definition of current limiting.

Time current curves are not really sufficient for making the type of comparison you are looking for as they are do not represent 'sub-cycle' performance (which is one reason '100% selective coordination' must be tested).
 
Last edited:
Solid state breakers (such as used in some military application and on the space station) can 'open' in < 40 nanoseconds. Much faster than any 'fuse'. Most of these use MOSFets, some use IGBTs, but those take a few hundred ns to 'open'.
 
Most of the time in regular commercial or industrial, the breaker will be some molded case thermal magnetic trip, not a current limiting electronic trip unit. In the large sizes like 600 amp 480 volt molded case, I never trust a breaker to trip fast enough without some type of damage, including damage to the breaker itself. You must have seen it, 480 volt breakers open under load or arcing faults and there's carbonization blown all over the breaker and sometimes coating the deadfront and panelboard cover.

The alternate choice to the molded case breaker is current limiting fuses, usually, not a current limiting breaker.

For semiconductors like a large inverter UPS output, protecting with a regular molded case breaker and paralleling, closing the inverter output into an unsynchronized condition, the IGBT's will blow out like fuses (going boom) but the breaker will not open. (Seen that, dangerous fakers). Semiconductor protecting fuses are very fast, probably the fastest available.

I've opened up a few spent molded case breaker with a hammer and the insides sometimes looks pretty hokey. In the large sizes like 600 amp, the breaker may see an arcing fault like a normal load and what trips is the upstream fuse, even the utility pole mounted fuse.

I generally credit the fuse with much greater sensitivity and calibration. In the same amp and volt rating you can get fast or slow fuses that are all current limiting, meaning they will open in less than 1/4 cycle before peak current. Very rare to see electronic trip current limiting breakers until you step up to the next class of equipment, like a Cutler Hammer DS, which are very impressive opening under load, but even those may have internal series fusible current limiters.

If you are comparing a non current limiting, non electronic trip breaker to a fuse, there's no comparison. They are different animals. Large molded case breakers can get pretty dangerous. The fuse is current limiting and much safer.

I expect a fuse to trip always, as specified, but consider myself lucky if the breaker is able to open without some type of catastrophic damage. The electronic trip current limiting breaker are pretty good, but the price will give the customer sticker shock.
 
__dan said:
Most of the time in regular commercial or industrial, the breaker will be some molded case thermal magnetic trip, not a current limiting electronic trip unit.
Click to expand...

Most of the current limiting molded case breakers I have seen are thermal magnetic.
The first one I sold was a 200A device back in 1978, well before molded case breakers routinely had electronic trip elements.
Breakers are like fuses, in that both are only intended to operate in their current limiting range once before they need to be replaced.

I see the issue as being that most operations of protective devices involve fault currents that are no where near the point of current limitation. This is one reason why arc flash studies use <100% of Ibf values when calculating incident energy. Without knowing the results for I?t and Ip, it is not really possible to compare devices (e.g. is a Class J fuse 'better' than an RK1?).
 
iwire said:
Unknown, the statements have been that fuse 'are better' 'are faster' 'provide better protection'.

You know, the typical vagueness of electricians. Like 'dirty' power. :D
Click to expand...

If you are in Boston and the breaker or fuse is in Worchester, they all trip the same. :happyyes:

If you are in the same plant and something is spitting blue flames and will not shut off, the electrician is the guy who has to run across the plant and dump the load. That's when you have to ask yourself, do you want to jump on the handle of a big metal enclosed fused disconnect or do you want to open a panelboard cover and throw the little handle of the molded case breaker, knowing some of those blue flames may leap out of that breaker right where your hand is. :happysad:

One fault type is a little slip of the screwdriver or a little metal tab in the wrong place. It pops and blows clear right away, but unfortunately sometimes there is a very nearby insulating busbar support and the carbonization from that little pop will deposit on the insulator busbar support and it starts to flash over. That's when breakers over 600 amp will see the arcing fault as a normal load and continue pumping into it as it flashes over the insulator. Could keep burning for a while until the utility fuse at the street opens. It's a problem that recurs. Really AFCI's should have started in big breakers and not the little residential ones.
 
__dan said:
If you are in the same plant and something is spitting blue flames and will not shut off, the electrician is the guy who has to run across the plant and dump the load. That's when you have to ask yourself, do you want to jump on the handle of a big metal enclosed fused disconnect or do you want to open a panelboard cover and throw the little handle of the molded case breaker, knowing some of those blue flames may leap out of that breaker right where your hand is. :happysad:
Click to expand...

Neither a fused disconect or a breaker is intended as a manual EPO. Manually opening either during a fault condition may be bad for your health.


One fault type is a little slip of the screwdriver or a little metal tab in the wrong place. It pops and blows clear right away, but unfortunately sometimes there is a very nearby insulating busbar support and the carbonization from that little pop will deposit on the insulator busbar support and it starts to flash over. That's when breakers over 600 amp will see the arcing fault as a normal load and continue pumping into it as it flashes over the insulator. Could keep burning for a while until the utility fuse at the street opens. It's a problem that recurs. Really AFCI's should have started in big breakers and not the little residential ones.
Click to expand...

Sorry I am missing the point you are trying to make here.
 
iwire said:
Sorry I am missing the point you are trying to make here.
Click to expand...

It's something I've seen or heard of closely at least two or three times. Arcing faults where the panelboard or a busway section itself is burning down and it continues to burn until the utility fuse at the street opens. The upstream feeder and service main breakers remain closed.

The sensitivity to arcing faults can be bad in some of the large size molded case breakers.
 
__dan said:
If you are in Boston and the breaker or fuse is in Worchester, they all trip the same. :happyyes:

If you are in the same plant and something is spitting blue flames and will not shut off, the electrician is the guy who has to run across the plant and dump the load. That's when you have to ask yourself, do you want to jump on the handle of a big metal enclosed fused disconnect or do you want to open a panelboard cover and throw the little handle of the molded case breaker, knowing some of those blue flames may leap out of that breaker right where your hand is. :happysad:

One fault type is a little slip of the screwdriver or a little metal tab in the wrong place. It pops and blows clear right away, but unfortunately sometimes there is a very nearby insulating busbar support and the carbonization from that little pop will deposit on the insulator busbar support and it starts to flash over. That's when breakers over 600 amp will see the arcing fault as a normal load and continue pumping into it as it flashes over the insulator. Could keep burning for a while until the utility fuse at the street opens. It's a problem that recurs. Really AFCI's should have started in big breakers and not the little residential ones.
Click to expand...
I do not see how the utility fuse could blow under an arcing fault that does not trip a breaker. Unless the fault is in the breaker, of course.


Sent from my XT1080 using Tapatalk
 
GoldDigger said:
I do not see how the utility fuse could blow under an arcing fault that does not trip a breaker. Unless the fault is in the breaker, of course.
Click to expand...

I had exactly the same reaction. Why did the busway feeder breaker or service main not open but the utility fuse did. Two incidents I was familiar with, I was told the equipment kept burning for more than 20 minutes. Guys had time to empty a few fire extinguishers
on it and it flared back up on them when the extinguishers were empty because the power was still on.

Both faults started as a small pop that blew clear immediately but the nearby busbar supporting insulator started flashing over from carbonization deposits. Then it just kept burning.

I don't know if i can find some literature on it with Google but I was expecting it was something others had also seen.

One of the incidents, my father lost the use of his right hand and it was my brothers on the fire extinguishers. The other, a large company I worked for, a 600 amp busway burned in half like a fuse, left a six inch gap clear across the busway. Both times the utility fuse was what opened.
 
Last edited:
GoldDigger said:
I wonder whether the main service breaker had not been maintained and had stuck or welded contacts?
Click to expand...

No. My take is the breakers were so large the arcing fault was below the sensitivity of the breaker to the problem. The fuse was more sensitive to an arcing fault. It is not something apparent just from the rating, the arcing fault is more dynamic and characteristic than x number of amps in a 60 Hz sinewave.
 
Status
Not open for further replies.
Top